My cat named kitty that is 11 months old ( found him on the streets last year when he was a kitten) hates to be inside of my house so he likes to be outside and go away for like a day or hours.
He just returned , he saw me and meows at me in a relaxed weird way. I just gave him his food but he had his tongue sticking out of his mouth and not drinking the fresh water that i gave him, he ate some food and lay down on his favorite spot outside. I had the feeling that something is not right because he had his nose wet and his paw too like of he has been licking it so long. ( i took some photos of it but i don't know how to send them)
For the cats that spent some days away, is common to come home and behave strangely as well as that they are thin and bruised. They should be left resting for a few days.
In case you see him really in a bad health, you can take him to the vet but in most cases they recover by themselves in a few days.
All the best.